. The Bell System technical journal . d to bring the magneti-zation approximately to the point N when no voice current is present,the maximum recording range cannot be obtained without excessiveamplitude distortion. For example if no bias is used, it has been found 174 BELL SYSTEM TECHNICAL JOURNAL that the output plotted against input will be too steep as shown in thecurve 1, Fig. 8. If the bias is too great, the output input curve will beinclined too much as shown in curve 2 of the same figure. When the o 30 3 20 10 ^ - V /- 3^ ^— / / / / 2. / / / ^ X / / ^ y / / y A / ^ y ^ / / y 20 30 40 5
. The Bell System technical journal . d to bring the magneti-zation approximately to the point N when no voice current is present,the maximum recording range cannot be obtained without excessiveamplitude distortion. For example if no bias is used, it has been found 174 BELL SYSTEM TECHNICAL JOURNAL that the output plotted against input will be too steep as shown in thecurve 1, Fig. 8. If the bias is too great, the output input curve will beinclined too much as shown in curve 2 of the same figure. When the o 30 3 20 10 ^ - V /- 3^ ^— / / / / 2. / / / ^ X / / ^ y / / y A / ^ y ^ / / y 20 30 40 50 INPUT IN DECIBELS 60 70 80 Fig. 8—The effect of the biasing current on the slope of the output-input level curves. proper value of bias is used the curve 3 is obtained which is inclinedat 45 degrees. Measurements of output versus input may thereforebe used to determine the proper bias current. Another method of determining the proper amount of bias is to plotthe output obtained from records of very weak signals as a function of. 0 12 3 4 BIAS CURRENT IN MILLIAMPERES Fig. 9—Effect of the biasing current on the intensity of weak signals. the bias current used during recording. Figure 9 shows such a curvefor a 1000-cycle record. In selecting the proper bias current from thecurve, the point at the crest gives the most efficient value but it is SOUND RECORDING ON MAGNETIC TAPE 175 better to favor some point slightly to the right of the crest in order toavoid the danger of very strong signals operating too far down on theleft side of the curve. The recording current for the 1000 cycles had avalue 30 db below the overload so that only a small amplitude wasused in obtaining the data for this curve. If the same set of pole-pieces is used both for recording and repro-ducing, there is no question of getting the reproducing pole-pieces inthe same alignment as the recording pole-pieces. If different sets areused care must be taken to get the same alignment. In order to keep the sign
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